LINE

#include <stdio.h>
#include <math.h>
#define NOL 2E-15

class line {
private:
    double qx,qy,qz;
    double x,y,z;
public:
    line(int);
    line(const line&);
    line(double x=0, double y=0, double z=0, double qx=1, double qy=1, double qz=1);
    void print(void);
    bool online(double, double, double);
    friend bool cross(line,line);
};
bool cross(line a, line b) {
    double x1,x2,y1,y2,z1,z2,qx,qy,qz,px,py,pz,A;
    x1=a.x;     x2=b.x;     px=b.qx;    qx=a.qx;
    y1=a.y;     y2=b.y;     py=b.qy;    qy=a.qy;
    z1=a.z;     z2=b.z;     pz=b.qz;    qz=a.qz;

    if(fabs(qx/px-qy/py) < NOL and fabs(qx/px-qz/pz) < NOL) return 0;
    A=qx*(py*(z2-z1)-pz*(y2-y1))-qy*(px*(z2-z1)-pz*(x2-x1))+qz*(px*(y2-y1)-py*(x2-x1));
    return (fabs(A) < NOL)  ?   1
                            :   0;

}

bool line::online(double xx, double yy, double zz) {
    double A,B,C;
    A=(x-xx)/qx;
    B=(y-yy)/qy;
    C=(z-zz)/qz;
    return (fabs(A-B)<NOL  &&  fabs(B-C)<NOL)   ?     1
                                                :     0;

}

void line::print(void) {
    printf("\n\nPryamaya: ");
    printf("\nx -%5.2lf     y -%5.2lf     z -%5.2lf",x,y,z);
    printf("\n---------- = ---------- = ----------");
    printf("\n%7.2lf%13.2lf%13.2lf",qx,qy,qz);
}

line::line(int) {
    printf("Vvedite Napravlyaushiy vector x,y,z: ");
    scanf("%lf%lf%lf",&qx,&qy,&qz);
    printf("Vvedite to4ky 4erez kotoryu proxodit pryamaya x,y,z: ");
    scanf("%lf%lf%lf",&x,&y,&z);
}
line::line(const line &A) {
    x=A.x;
    y=A.y;
    z=A.z;
    qx=A.qx;
    qy=A.qy;
    qz=A.qz;
}
line::line(double xx, double yy, double zz, double qxx, double qyy, double qzz) {
    x=xx;
    y=yy;
    z=zz;
    qx=qxx;
    qy=qyy;
    qz=qzz;
}

int main() {
    line a(-2,0,1,2,-3,4);
    line b(3,1,7,3,4,2);
    if(a.online(0,0,0)) printf("YES ON_LINE\n");
    if(cross(a,b)) printf("YES CROSS");
}